PROJECT SUMMARY First-line pharmacotherapies for major depressive disorder (MDD) are only moderately successful. With anodal stimulation of the left dorsolateral prefrontal cortex (DLPFC), transcranial direct current stimulation (tDCS), a low-intensity neuromodulation technique of minimal risk, may elicit antidepressant effects and improve cognitive control in individuals with MDD. Prior evidence suggests that prefrontal-limbic circuits, including the DLPFC and dorsomedial anterior cingulate cortex (dACC), are involved in the regulation of mood and emotion. Their modulation by tDCS may thus contribute to antidepressant effects. However, the extent to which these regions are engaged by tDCS and how their recruitment contributes to clinical response is unknown. Response to tDCS also remains mixed at the individual level where the size, location and intensity of stimulation might account for varied therapeutic effects. Recently, high definition (HD) tDCS has been developed that allows for more focal neural stimulation. During the R61 phase of this Exploratory Clinical Trial we aim to apply and compare HD-tDCS, conventional tDCS (C-tDCS) and sham tDCS in patients with moderate to severe MDD (N=60, n=20 in each group) to test for differences in the engagement of mood regulating DLPFC and dACC regions between conditions using a randomized, double blind design. We will use MRI-guided stereotaxy to optimize and standardize DLPFC electrode placement and novel MRI techniques with concurrent tDCS to test the regional distribution of tDCS current at different stimulation intensities. We will use 3D GRASE pseudo- continuous arterial spin labeling (pCASL) MRI, compared before and after subjects complete 12 daily 30 minute sessions of 2 mA stimulation, to test for tDCS-related changes in regional cerebral blood flow (rCBF) in prefrontal circuitry. Significant tDCS induced magnetic field changes in DLPFC and significant changes in rCBF between baseline and the end of the tDCS trial in the DLPFC and dACC for either of the active tDCS conditions compared to sham will constitute the go-no-go criterion for proceeding to the R33 Phase. Using the active tDCS modality showing greater target engagement and neurophysiological effects without peripheral side effects, the R33 Phase will randomize patients with moderate to severe MDD (N=100, n=50 in each group) to active or sham left anodal DLPFC tDCS. Patients will again complete MRI scans including tDCS- current mapping and pCASL as well as two functional imaging tasks probing cognitive control and emotion negativity bias, recruiting prefrontal-limbic circuitry, before and after completing a 12-day trial of 30-minute tDCS sessions. Demonstration that target engagement with active tDCS varies in association with improved mood (primary outcome), cognitive function and changes in task-related brain activation (secondary outcomes) will constitute the criterion for continued R01 investigation. Results of this trial are expected to lead to an optimized MRI guided tDCS treatment of MDD and deepen understanding of the physiological mechanisms of tDCS therapy.